Ascorbic acid (or L-ascorbic acid) is the most common form of vitamin C, and it acts as a powerful antioxidant to fight pigmentation, maintain skin barrier function, and smooth skin. This form is the most biologically active, which is why it's so heavily used in skincare. The downside of ascorbic acid is that it can degrade quickly when exposed to UV light or air (think an apple turning brown) and become ineffective. It is more powerful when paired with stabilizing ingredients like Vitamin E or ferulic acid.